DESCRIPTION

The
msgsnd()
and
msgrcv()
system calls are used, respectively, to send messages to,
and receive messages from, a message queue.
The calling process must have write permission on the message queue
in order to send a message, and read permission to receive a message.

The
msgp
argument is a pointer to caller-defined structure
of the following general form:

The
mtext
field is an array (or other structure) whose size is specified by
msgsz,
a nonnegative integer value.
Messages of zero length (i.e., no
mtext
field) are permitted.
The
mtype
field must have a strictly positive integer value.
This value can be
used by the receiving process for message selection
(see the description of
msgrcv()
below).

msgsnd()

The
msgsnd()
system call appends a copy of the message pointed to by
msgp
to the message queue whose identifier is specified
by
msqid.

If sufficient space is available in the queue,
msgsnd()
succeeds immediately.
(The queue capacity is defined by the
msg_qbytes
field in the associated data structure for the message queue.
During queue creation this field is initialized to
MSGMNB
bytes, but this limit can be modified using
msgctl(2).)
If insufficient space is available in the queue, then the default
behavior of
msgsnd()
is to block until space becomes available.
If
IPC_NOWAIT
is specified in
msgflg,
then the call instead fails with the error
EAGAIN.

A blocked
msgsnd()
call may also fail if:

*

the queue is removed,
in which case the system call fails with
errno
set to
EIDRM;
or

*

a signal is caught, in which case the system call fails
with
errno
set to
EINTR;seesignal(7).
(msgsnd()
is never automatically restarted after being interrupted by a
signal handler, regardless of the setting of the
SA_RESTART
flag when establishing a signal handler.)

Upon successful completion the message queue data structure is updated
as follows:

msg_lspid
is set to the process ID of the calling process.

msg_qnum
is incremented by 1.

msg_stime
is set to the current time.

msgrcv()

The
msgrcv()
system call removes a message from the queue specified by
msqid
and places it in the buffer
pointed to by
msgp.

The argument
msgsz
specifies the maximum size in bytes for the member
mtext
of the structure pointed to by the
msgp
argument.
If the message text has length greater than
msgsz,
then the behavior depends on whether
MSG_NOERROR
is specified in
msgflg.
If
MSG_NOERROR
is specified, then
the message text will be truncated (and the truncated part will be
lost); if
MSG_NOERROR
is not specified, then
the message isn't removed from the queue and
the system call fails returning -1 with
errno
set to
E2BIG.

The argument
msgtyp
specifies the type of message requested as follows:

*

If
msgtyp
is 0,
then the first message in the queue is read.

*

If
msgtyp
is greater than 0,
then the first message in the queue of type
msgtyp
is read, unless
MSG_EXCEPT
was specified in
msgflg,
in which case
the first message in the queue of type not equal to
msgtyp
will be read.

*

If
msgtyp
is less than 0,
then the first message in the queue with the lowest type less than or
equal to the absolute value of
msgtyp
will be read.

The
msgflg
argument is a bit mask constructed by ORing together zero or more
of the following flags:

IPC_NOWAIT

Return immediately if no message of the requested type is in the queue.
The system call fails with
errno
set to
ENOMSG.

MSG_EXCEPT

Used with
msgtyp
greater than 0
to read the first message in the queue with message type that differs
from
msgtyp.

MSG_NOERROR

To truncate the message text if longer than
msgsz
bytes.

If no message of the requested type is available and
IPC_NOWAIT
isn't specified in
msgflg,
the calling process is blocked until one of the following conditions occurs:

*

A message of the desired type is placed in the queue.

*

The message queue is removed from the system.
In this case the system call fails with
errno
set to
EIDRM.

*

The calling process catches a signal.
In this case the system call fails with
errno
set to
EINTR.
(msgrcv()
is never automatically restarted after being interrupted by a
signal handler, regardless of the setting of the
SA_RESTART
flag when establishing a signal handler.)

Upon successful completion the message queue data structure is updated
as follows:

msg_lrpid
is set to the process ID of the calling process.

msg_qnum
is decremented by 1.

msg_rtime
is set to the current time.

RETURN VALUE

On failure both functions return -1
with
errno
indicating the error,
otherwise
msgsnd()
returns 0
and
msgrcv()
returns the number of bytes actually copied into the
mtext
array.

ERRORS

When
msgsnd()
fails,
errno
will be set to one among the following values:

EACCES

The calling process does not have write permission on the message queue,
and does not have the
CAP_IPC_OWNER
capability.

EAGAIN

The message can't be sent due to the
msg_qbytes
limit for the queue and
IPC_NOWAIT
was specified in
msgflg.

EFAULT

The address pointed to by
msgp
isn't accessible.

EIDRM

The message queue was removed.

EINTR

Sleeping on a full message queue condition, the process caught a signal.

EINVAL

Invalid
msqid
value, or nonpositive
mtype
value, or
invalid
msgsz
value (less than 0 or greater than the system value
MSGMAX).

ENOMEM

The system does not have enough memory to make a copy of the
message pointed to by
msgp.

When
msgrcv()
fails,
errno
will be set to one among the following values:

E2BIG

The message text length is greater than
msgsz
and
MSG_NOERROR
isn't specified in
msgflg.

EACCES

The calling process does not have read permission on the message queue,
and does not have the
CAP_IPC_OWNER
capability.

EAGAIN

No message was available in the queue and
IPC_NOWAIT
was specified in
msgflg.

EFAULT

The address pointed to by
msgp
isn't accessible.

EIDRM

While the process was sleeping to receive a message,
the message queue was removed.

EINTR

While the process was sleeping to receive a message,
the process caught a signal; see
signal(7).

EINVAL

msgqid
was invalid, or
msgsz
was less than 0.

ENOMSG

IPC_NOWAIT
was specified in
msgflg
and no message of the requested type existed on the message queue.

CONFORMING TO

SVr4, POSIX.1-2001.

NOTES

The
msgp
argument is declared as struct msgbuf * with
libc4, libc5, glibc 2.0, glibc 2.1.
It is declared as void *
with glibc 2.2 and later, as required by SUSv2 and SUSv3.

The following limits on message queue resources affect the
msgsnd()
call:

MSGMAX

Maximum size for a message text: 8192 bytes
(on Linux, this limit can be read and modified via
/proc/sys/kernel/msgmax).

MSGMNB

Default maximum size in bytes of a message queue: 16384 bytes
(on Linux, this limit can be read and modified via
/proc/sys/kernel/msgmnb).
The superuser can increase the size of a message queue beyond
MSGMNB
by a
msgctl(2)
system call.

The implementation has no intrinsic limits for the system wide maximum
number of message headers
(MSGTQL)
and for the system wide maximum size in bytes of the message pool
(MSGPOOL).